Gateway device and port number assignment method

ABSTRACT

When streaming transfer of voice or image is performed using a real time streaming protocol (RTSP), streaming transfer of a high bit rate is enabled even in an environment of using network address port translation (NAPT) and network address translation-protocol translation (NAT-PT). When a SETUP message of RTSP is received, a port number having the same number of digits as a client port number notified by a client is assigned from a port number pool, and is registered in an NAPT table. The SETUP message is rewritten with the port number.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2009-257722, filed on Nov. 11, 2009, the content of which ishereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a gateway device and a port numberassignment method, and more particularly, to a gateway device and a portnumber assignment method using a real-time packet.

BACKGROUND OF THE INVENTION

Currently, to share a single global address with a plurality ofterminals installed in a home, a company, and the like, a networkaddress port translation (NAPT) is widely used (see “IP Network AddressTranslator (NAT) Terminology and Considerations” by P. Srisuresh et al.,in August, 1999, IETF,<http://www.ietf.org/rfc/rfc2663.txt?number=2663>).

Also, a network address translation-protocol translation (NAT-PT) isknown as a method of interconnecting networks using different protocols,more particularly, interconnecting a network using Internet protocolversion 4 (IPv4) and a network using Internet protocol version 6 (IPv6),as a protocol (see “Network Address Translation—Protocol Translation(NAT-PT)” by G. Tsirtsis et al., in February, 2000, IETF,<http://www.ietf.org/rfc/rfc2766.txt?number=2766>).

These NAPT and NAT-PT enable interconnection between different networksby appropriately converting a port number of an IP header and atransmission control protocol (TCP)/user datagram protocol (UDP) header.However, in addition to the IP header and the TCP/UDP header, an addressand a port number are described in a payload according to anapplication. In this case, when realizing NAPT and NAT-PT, there is aneed to rewrite an address and port number information in the payloadpart in addition to the IP header and the TCP/UDP header.

The above conversion function is referred to as an application levelgateway (ALG). Even in the case of the ALG, a position where the addressor the port number is described is different in the payload according tothe application. Accordingly, a rewrite rule needs to be defined foreach application.

Specifically, a session initiation protocol (SIP) message needs rewriteof the payload part. JP-A-2005-073302 enables interconnection using theSIP message in an environment of using NAT-PT by converting the SIPmessage.

In the meantime, a real time streaming protocol (RTSP) is a protocolcontrolling streaming delivery of a voice, a video signal, and the like,and also corresponds to a protocol where information needing conversionis written in the payload part. In the case of the RTSP, a client portnumber to be used for a real-time transport protocol (RTP)/RTP controlprotocol (RTCP) is described in a message of a SETUP method.Accordingly, the SETUP message needs to be rewritten by an RTSP-ALG inan environment where NAPT processing or NAT-PT is performed.

In this case, when a number of digits of a client port numbertransferred to the payload by a client is different from a number ofdigits of a port number assigned by NAPT processing, a segment length ofTCP is different with respect to before conversion and after conversion.

When the segment length of TCP changes, an acknowledgment number withrespect to a corresponding message and a sequence number of a subsequentpacket may also change. This is because a value in which the TCP segmentlength is added to the sequence number is used as the acknowledgementnumber. Also, this is because a value in which this TCP segment lengthis added to a previous sequence number is used as the sequence number.

To consistently maintain a session, a converter for performing NAPTprocessing or NAT-PT needs to have a difference list with respect to thesequence number and the acknowledgement number, and then needs torewrite the sequence number and the acknowledgement number with respectto a packet received thereafter (see “network addresstranslation-protocol translation (NAT-PT)”). Once the segment length ofTCP changes, it may affect a sequence number of a subsequent TCP packet.Accordingly, the above conversion processing is required until thesession is terminated.

In the case of RTSP, two types of deliveries such as UDP delivery andinterleave (IL) delivery are specified as a delivery method. Among them,the IL delivery superimposes RTP/RTCP data on a TCP session for an RTSPcontrol message. Accordingly, when the segment length changes,conversion cost of the sequence number and the acknowledgement numbermay increase significantly.

JP-A-2009-015392 discloses a study that may not decrease a transfer ratewhen a packet is lost by transmitting a proxy ACK in addition to theaforementioned difference list.

SUMMARY OF THE INVENTION

According to techniques disclosed in “Network AddressTranslation-Protocol Translation (NAT-PT)” and JP-A-2009-015392, once asegment length changes due to rewrite of a real time streaming protocol(RTSP) SETUP message, rewrite processing of a sequence number and anacknowledgement number may be continued with respect to a subsequentpacket. As a result, conversion cost of the sequence number and theacknowledgement number may be used until a session is terminated.

The present invention relates to a technique that may not change asegment length when an RTSP SETUP message is rewritten. As a result, thepresent invention provides a gateway device and a port number assignmentmethod that may have no need to rewrite a sequence number and anacknowledgment number of a subsequent packet.

To achieve the aforementioned objects, according to an aspect of thepresent invention, there is provided a gateway device including a packetdistribution processing unit, a header converting unit, and a transferprocessing unit, and converting a first Internet protocol (IP) addressand a second IP address by referring to an IP address conversion table.The gateway device may further include a payload IP address convertingunit. The payload IP address converting unit may acquire a number ofdigits of a second port number included in a payload of a controlmessage, convert the second port number to a first port number havingthe same number of digits, store the second port number and the firstport number in the IP address conversion table, and convert, to thesecond port number, the first port number included in a response messagewith respect to the control message.

According to another aspect of the present invention, there is provideda method for assigning a port number, including the steps of: receivinga message to determine a destination IP address; determining anapplication protocol; determining a method; determining a transferdirection; when the transfer direction is upstream, acquiring a numberof digits of a request port number described in a payload of themessage, assigning a port number having the same number of digits, andrecording the assigned port number in the IP address conversion table;when the transfer direction is downstream, acquiring a request portnumber from the IP address conversion table; updating the payload; andconverting an IP address and a port number described in a header of themessage.

According to the embodiments of the present invention, since a segmentlength does not change in the case of rewriting an RTSP SETUP messageeven in an environment of using network address port translation(NAPT)/NAT-PT, there is no need to rewrite a sequence number and anacknowledgement number of transmission control protocol (TCP).Accordingly, it is possible to prevent deterioration of transfercapability occurring due to rewrite processing. Also, complex processingin consideration of a case where a difference list and a packet are lostis not required.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which;

FIG. 1 is a hardware block diagram of a network system;

FIG. 2A is a diagram to describe a SETUP packet and a response packetthereof;

FIG. 2B is a diagram to describe a SETUP packet and a response packetthereof;

FIG. 2C is a diagram to describe a SETUP packet and a response packetthereof;

FIG. 2D is a diagram to describe a SETUP packet and a response packetthereof;

FIG. 3 is a diagram to describe a network address port translation(NAPT) table;

FIG. 4 is a block diagram to describe a configuration of a gatewaydevice;

FIG. 5 is a flowchart to describe processing of a gateway device;

FIG. 6 is a flowchart to describe gateway processing;

FIG. 7 is a sequence diagram from an RTSP session establishment todisconnection;

FIG. 8A shows a real time streaming protocol (RTSP) SETUP message(four-digit port number);

FIG. 8B shows an RTSP SETUP response message (four-digit port number);

FIG. 9A shows an RTSP SETUP message (five-digit port number);

FIG. 9B shows an RTSP SETUP response message (five-digit port number);and

FIG. 10 is a sequence diagram to describe a sequence number and anacknowledgement number before and after a message rewrite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Like elements refer to likereference numerals shown in the drawings, and the descriptions are notrepeated.

Initially, a network system 500 in which a gateway device 200 is appliedwill be described with reference to FIG. 1. In FIG. 1, the networksystem 500 includes a plurality of terminals 100 connected to a localarea network (LAN) 102, the gateway device 200 connecting the LAN 102and a wide area network (WAN) 103, and a contents server 300 connectedto the WAN 300.

Private Internet protocol (IP) addresses 113-1 to 113-3 are assigned toterminals 100-1 to 100-3. Meanwhile, the WAN 103 is connected to theInternet and a regional network where a communication carrier isinstalled, and uses a global IP address. When the terminals 100-1 to100-3 belonging to the LAN side network 102 access the contents server300 of the WAN side network 103, the terminals 100-1 to 100-3 mayperform communication by address and port conversion using a networkaddress translation (NAT) function.

A SETUP message of real time streaming protocol (RTSP) will be describedwith reference to FIG. 2. FIG. 2 briefly shows an IP header 109 of amessage 108, a transmission control protocol (TCP) header 110, and amessage 111 of a data part. In FIG. 2A, a SETUP message 108-1 istransmitted from the terminal 100-1 to the contents server 300.

In FIG. 2B, when the packet 108-1 is received, the gateway device 200converts Src IP (source IP address) 192.168.1.10 to IP address 2.2.2.2of WAN side interface (I/F) 113-4 of the gateway device 200, in order totransfer the packet 108-1 to the contents server 300 belonging to theWAN 103. In addition, Src port (source port number) 6001 is alsoconverted to a port number 40001 secured by the gateway device 200.

In this case, the gateway device 200 stores a corresponding relation ina NAPT table 220. Even when the gateway device 200 receives a responsemessage 108-3 from the contents server 300, the gateway device 200reversely converts Dst IP 2.2.2.2 (destination IP address) to192.168.1.10, and converts Dst Port (destination port number) 40001 to6001 by referring to the above corresponding relation. As a result, thepacket reaches the terminal 100-1. A basic operation of NAPT has beendescribed so far. FIG. 2C and FIG. 2D show a response message from thecontents server 300 and a reversely converted message thereof.

However, when a SETUP message 111 is further received, the gatewaydevice 200 registers a set of a conversion port number, a source IPaddress, and a conversion IP address with a number (a previous or nextnumber when separable by “-”) following “client_Port=” of the data part,in the NAPT table 220. In the case of the conversion port number to beassigned, a port number having the same number of digits as the notifiedport number is assigned, which will be described below. Through this, adata length of the data part does not change and thus, a sequence numberand an acknowledgment number in TCP will not be affected.

The NAPT table 220 will be described with reference to FIG. 3. In FIG.3, the NAPT table 220 includes a source IP address 221, a source portnumber 222, a conversion IP address 223, a conversion port number 224,and a final use date and time 225.

The source IP address 221 is a private IP address used for the LAN 102.The source port number 222 is a port number of the terminal 100-1connected to the LAN 102. The conversion IP address 223 is a WAN side IPaddress of the gateway device 200. The conversion port number 224 is aWAN side port number of the gateway device 200. The final use date andtime 225 is a date and time when a corresponding entry is finallygenerated or referred to. The final use date and time 225 is used fordeletion of the corresponding entry.

When a first departure packet from LAN to WAN is received, and when RTSPSETUP is received, an entry is added. In the case of a subsequentlyreceived packet, an upstream packet refers to the conversion IP address223 and the conversion port number 224 using, as a key, the source IPaddress 221 and the source port number 222. A downstream packet refersto the source IP address 221 and the source port number 222 using, as akey, the conversion IP address 223 and the conversion port number 224.An entry not used for a predetermined amount of time is deleted throughan aging process.

Entries 2, 3, 6, and 7 emphasized in FIG. 3 are entries registered byRTSP-application level gateway (ALG) processing. Since a source portnumber of each of the entries 2 and 3 is of four digits, a four-digitport number is assigned as a conversion port number. Since a source portnumber of each of the entries 6 and 7 is of five digits, a five-digitport number is assigned as the conversion port number. Entries 1, 4, 5,and 8 entered by general NAPT processing may be assigned with theconversion port number regardless of a number of digits of the sourceport number. Accordingly, empty port numbers are sequentially assignedregardless of the number of digits.

A configuration of the gateway device 200 will be described withreference to FIG. 4. The gateway device 200 includes a packet receptionprocessing unit 201, a packet distribution processing unit 202, atransfer processing unit 203, a packet transmission processing unit 204,a NAPT processing unit 205, an RTSP-ALG processing unit 206, aconversion port number pool 207, an aging processing unit 208, and aNAPT table 220.

The packet reception processing unit 201 of LAN I/F or WAN I/F receivesa packet from LAN or WAN. The packet reception processing unit 201transfers the received packet to the packet distribution processing unit202. The packet distribution processing unit 202 distributes the packetto the transfer processing unit 203, the NAPT processing unit 205, orthe RTSP-ALG processing unit 206 based on contents of the packet. WhenNAPT processing such as transferring within the same network is notrequired, the packet distribution processing unit 202 distributes thepacket to the transfer processing unit 203. When the packet istransmitted from a terminal within the LAN side network 102 to a serverwithin the WAN side network 103, the packet distribution processing unit202 distributes the packet to the NAPT processing unit 205.

Initially, the NAPT processing unit 205 refers to the NAPT table 220.When no entry exists, the NAPT processing unit 205 is assigned with aport number available as a conversion port number from the conversionport number pool 207, as an initially passing packet. The NAPTprocessing unit 205 rewrites an IP address and a port number included inan IP header and a TCP header. At the same time, the NAPT processingunit 205 makes an entry in the NAPT table 220. With respect to a packetreceived after a second time and a response packet from the server, theNAPT processing unit 205 rewrites and thereby transfers the packet byreferring to the NAPT table 220.

In particular, when the packet received by the packet distributionprocessing unit 202 is a SETUP message of RTSP, the packet distributionprocessing unit 202 distributes the packet to the RTSP-ALG processingunit 206. The RTSP-ALG processing unit 206 detects the port number usedin real-time transport protocol (RTP) and RTP control protocol (RTCP)based on contents written in the message. The RTSP-ALG processing unit206 assigns a port number having the same number of digits as thedetected port number from the conversion port number pool 207, andrewrites the message. The RTSP-ALG processing unit 206 registers therewritten message in the NAPT table 220. When a SETUP response messageis received, reversely, the RTSP-ALG processing unit 206 rewrites themessage by referring to the NAPT table 220. The RTSP-ALG processing unit206 transmits the message to the NAPT processing unit 205. The NAPTprocessing unit 205 performs NAPT processing and transfers the messageto the transfer processing unit 203.

The aging processing unit 208 refers to the final use date and time 225of the NAPT table 220 at predetermined cycles. The aging processing unit208 deletes, from the NAPT table 220, an entry not used for apredetermined amount of time, and returns the assigned port number tothe conversion port number pool 207.

A processing flow of the gateway device 200 will be described withreference to FIG. 5. This flow is initiated by packet reception. When apacket is received, the gateway device 200 determines whether thereceived packet corresponds to NAPT target by referring to a destinationIP address (S401). When the received packet does not correspond to anNAPT target address, the gateway device 200 performs general transferprocessing (S402) and then terminates. When the received packetcorresponds to the NAPT target address in step 401, the gateway device200 determines an application protocol (S403). In this case, except forRTSP (destination and source port numbers are 554), the gateway device200 performs NAPT processing (S404). Specifically, in the case of anupstream packet, NAPT processing performs source address conversion andsource port conversion. On the other hand, in the case of a downstreampacket, NAPT processing performs destination address conversion anddestination port conversion. Where, upstream herein is communicationfrom the terminal 100 to the contents server 300, and downstream iscommunication from the contents server 300 to the terminal 100.

Conversely, when the application protocol is determined as the RTSP instep 403, the gateway device 200 further determines an RTSP method(S405). Specifically, the gateway device 200 determines whether themethod is SETUP. When the method is determined as a method excluding theSETUP method, the gateway device 200 goes to step 404. When the methodis determined as the SETUP method (including Reply) in step 405, thegateway device 200 executes gateway processing (S406) and goes to step404.

The gateway processing (S406) will be described with reference to FIG.6. The RTSP-ALG processing unit 206 determines a transfer direction(S501). In the case of upstream direction, the RTSP-ALG processing unit206 reads a number of digits of a request port from a SETUP message(S502). The RTSP-ALG processing unit 206 assigns a port number havingthe same number of digits (S503). Next, the RTSP-ALG processing unit 206makes an entry in the NAPT table 220 by making, as a set, a sourceaddress, a source port number, and a conversion address with theassigned port number. Finally, the RTSP-ALG processing unit 206 rewritesthe message with the assigned port number (S505). Meanwhile, in the caseof downstream direction in step 501, the RTSP-ALG processing unit 206goes to step 505 by searching and referring to the NAPT table 220(S506).

A sequence from RTSP session establishment to termination in anenvironment using the gateway device 200 will be described withreference to FIG. 7. In RTSP, the terminal 100-1 and the contents server300 initially exchange available function information using an OPTIONmethod. Next, the terminal 100-1 acquires contents information using aDESCRIBE method. In addition, the terminal 100-1 determines atransmission scheme using a SETUP method. Hereinafter, the abovesequence will be described in detail.

The terminal 100-1 transmits an OPTION message describing an availablefunction to the gateway device 200 (S601). The gateway device 200converts a source IP address and a source port number and transmits theOPTION message to the contents server 300 (S602). The contents server300 transmits 200 OK describing an available function to the gatewaydevice 200 (S603). The gateway device 200 converts a destination IPaddress and a destination port number and transmits 200 OK to theterminal 100-1 (S604).

The terminal 100-1 transmits a DESCRIBE message for acquiring contentsinformation to the gateway device 200 (S606). The gateway device 200converts a source IP address and a source port number and transmits theDESCRIBE message to the contents server 300 (S607). The contents server300 transmits 200 OK describing contents information to the gatewaydevice (S608). The gateway device 200 converts a destination IP addressand a destination port number and transmits 200 OK to the terminal 100-1(S609).

The terminal 100-1 transmits a SETUP message describing availableTransport to the gateway device 200 (S611). The gateway device 200 readsa source port number from the SETUP message and converts the read sourcenumber to a port number having the same number of digits (S612). Thegateway device 200 converts a source IP address and a source port numberof a header and transmits the SETUP message to the contents server 300(S613). The contents server 300 transmits 200 OK describing selectedTransport to the gateway device 200 (S614). The gateway device 200 readsa destination port number from 200 OK, and converts the read destinationport number to an original source port number (S616). The gateway device200 converts the destination IP address and the destination port numberand transmits 200 OK to the terminal 100-1 (S617). The terminal 100-1transmits a PLAY message to the gateway device 200 (S618). The gatewaydevice 200 converts a source IP address and a source port number, andtransmits the PLAY message to the contents server 300 (S619). Thecontents server 300 transmits 200 OK to the gateway device 200 (S621).The gateway device 200 converts a destination IP address and adestination port number, and transmits 200 OK to the terminal 100-1(S622).

The terminal 100-1 and the contents server 300 start exchanging of anRTP packet and an RTCP packet. Subsequently, the terminal 100-1transmits, to the gateway device 200, a TEARDOWN message that is an endof reproduction (S623). The gateway device 200 converts a source IPaddress and a source port number and transmits the TEARDOWN message tothe contents server 300 (S624). The contents server 300 transmits 200 OKto the gateway device 200 (S626). The gateway device 200 converts adestination IP address and a destination port number and transmits 200OK to the terminal 100-1 (S627).

In step 611, the terminal (RTSP client) 100-1 notifies a port number tobe used. However, since the port number is changed in an NATenvironment, the gateway device 200 rewrites a message to a conversionport number having the same number of digits. The gateway device 200also rewrites a response message (200 OK) by referring to the NAPT table220. Reproduction of contents uses the PLAY method. Next, streamingtransfer is performed by the RTP packet and RTCP packets using atransmission method determined by the SETUP method. Finally,reproduction of contents is terminated by the TEARDOWN method.

Contents of the message to be rewritten will be described with referenceto FIG. 8A, FIG. 8B, FIG. 9A, and FIG. 9B. FIG. 8A(a) shows a message108A-1 of the SETUP method transmitted by the terminal 100-1. Meanwhile,FIG. 8A(b) shows a message 108A-2 of the SETUP message transmitted bythe gateway device 200. In FIG. 8A, the SETUP method may be determineddepending on whether a character string of “SETUP” exists in a frontpart of a payload. A numerical value (indicating a range when separableby “-”) following “client_port=” of the payload of this messageindicates the source port number of the terminal side 100-1 used in RTPand RTCP. The gateway device 200 acquires a number of digits of thisport number and assigns a conversion port number having the same numberof digits, and rewrites the message in FIG. 8A(b).

FIG. 8B(a) shows a response message 108A-3 of the SETUP methodtransmitted by the contents server 300. FIG. 8B(b) shows a responsemessage 108A-4 of the SETUP method transmitted by the gateway device200. In FIG. 8B(a), since “client_port=” is described in this message,the gateway device 200 reversely converts a subsequent value byreferring to the NAPT table 220.

FIG. 8A and FIG. 8B show a case where a port number has four digits.FIG. 9A and FIG. 9B show a case where the port number has five digits.

A sequence number and an acknowledgement number before and after amessage rewrite will be described with reference to FIG. 10. To explainplainly, FIG. 10 shows a circumstance where data only from a client to aserver is described. Also, a round value is used as a segment length.

In FIG. 10, the terminal 100-1 initially transmits a packet of a segmentlength (Len=) 100 to the contents server 300 (S901). In this case, asequence number (Seq=) is 10000. The gateway device 200 converts asource IP address and a source port number and transfers the packet tothe contents server 300 (S902). When the packet is received, thecontents server 300 returns Ack of acknowledgment number (Ack=) 10100 inorder to request subsequent data (S903). The gateway device 200 convertsa destination IP address and a destination port number and transfers Ackto the terminal 100-1 (S904).

Next, the terminal 100-1 transmits a SETUP message 903 of the segmentlength 100 (S906). The gateway device 200 rewrites the message (S907).The gateway device 200 transmits the rewritten message to the contentsserver 300 (S908). In this case, when the message is rewritten, thenumber of digits of the port number is matched and thus, the segmentlength does not change.

However, if the segment length changes, the gateway device 200 may needto rewrite the sequence number and the acknowledgement number indicatedby the square within the range indicated in FIG. 10 in order to maintaina consistency of TCP sequence. Also, since the influence on all thesubsequent acknowledgement numbers of sequence and sequence numbers isexerted, there is a need to rewrite the sequence number and theacknowledgement number.

When the packet is received, the contents server 300 returns Ack of theacknowledgement number (Ack=) 10200 in order to request subsequent data(S909). The gateway device 200 converts the destination IP address andthe destination port number and transfers Ack to the terminal 100-1(S911). The terminal 100-1 transmits the packet of the segment length(Len=) 100 to the contents server 300 (S912). In this case, the sequencenumber (Seq=) is 10200 which is the same as the received acknowledgementnumber. The gateway device 200 converts the source IP address and thesource port number and transfers the packet to the contents server 300(S913). When the packet is received, the contents server 300 returns Ackof the acknowledgement number (Ack=) 10300 in order to requestsubsequent data (S914). The gateway device 200 converts the destinationIP address and the destination port number and transfers Ack to theterminal 100-1 (S916).

According to the present embodiment, it is guaranteed that a port numberhaving the same number of digits is assigned as the conversion portnumber. Specifically, it is guaranteed that the segment length after amessage rewrite does not change. Accordingly, with respect to afollowing sequence, there is no need to rewrite the acknowledgementnumber and the sequence number.

What is claimed is:
 1. A gateway device comprising: a packetdistribution processing unit, a header converting unit, a transferprocessing unit, and a payload IP address converting unit, wherein thegateway device is configured to convert a first Internet protocol (IP)address and a second IP address by referring to an IP address conversiontable, wherein the payload IP address converting unit is configured to:acquire a number of digits of a second port number included in a payloadof a control message; convert the second port number to a first portnumber having the same number of digits; store the second port numberand the first port number in the IP address conversion table; convert,to the second port number, the first port number included in a payloadof a response message with respect to the control message; and equalizesegment lengths by matching the number of digits of the port numbersduring the conversion.
 2. The gateway device according to claim 1,further comprising: a conversion port number pool, wherein theconversion port number pool comprises the first port number of aplurality of numbers of digits, and wherein the payload IP addressconverting unit is assigned the first port number from the conversionport number pool.
 3. The gateway device according to claim 2, furthercomprising: an aging processing unit, wherein the IP address conversiontable records finally used time information in each record, and whereinthe aging processing unit is configured to return, to the conversionport number pool, the first port number recorded in a correspondingrecord based on the time information.
 4. A method for assigning a portnumber, comprising the steps, performed by a gateway device, of:receiving a message to determine a destination IP address; determiningan application protocol; determining a method; determining a transferdirection; when the transfer direction is upstream: acquiring a numberof digits of a request port number described in a payload of themessage, assigning a port number having the same number of digits, andrecording the assigned port number in the IP address conversion table;and when the forwarding direction downstream: acquiring a request portnumber from the IP address conversion table; updating the payload;converting an IP address and a port number described in a header of themessage; and equalizing segment lengths by matching the number of digitsof the port numbers during the conversion.